Kinematic analysis of volunteers in a highly reclined rigid seat in limited load frontal sled tests

The goal of the study was to investigate the kinematic response patterns of human volunteers in highly reclined postures with a safe limited load. The sled testing environment consisted of an adjustable rigid seat and an integrated 3-point seat belt, using a pulse with a nominal peak deceleration of 3.5 g. Preliminary tests with anthropomorphic test devices and simulations with human body model were performed to verify the safety of the testing environment. Various sensors were set up to record static data and kinematic responses from three 50th percentile male volunteers. A total of 36 tests were carried out under 4 seat configurations, including standard posture, semi-reclined posture, reclined posture, and zero-gravity posture (a modern term for a highly reclined vehicle seat design mimicking a comfortable recliner with leg support). All procedures were approved by the relevant ethics committees. The results indicated that as the reclining degree increased, the initial position of the hip moved backward and downward. The maximum displacement in the Z-axis of the head and neck increased, as well as the forward excursion of the upper torso and hip also significantly increased, while the shoulder and lap belts forces decreased. This illustrates that the integrated 3-point seat belt fails to effectively restrain the torso and hip of the occupants in highly reclined postures, particularly in the zero-gravity posture. These responses mirror those of a real human body in the early stage of a collision, providing insights into the potential injury risks for reclined occupants in crash.